Figure 49: Typical Three-Terminal Plain Feeder Circuit - GE MiCOM P40 Agile Technical Manual

Chapter 6 - Current Differential Protection
These settings will give a characteristic suitable for most applications. It leaves only the Phase Is1 setting to be
decided by you. The value of this setting should be in excess of any mismatch between line ends. It should also
account for line charging current, where necessary.
By considering the circuit shown below, the settings for the phase current differential element can be established.
End A
275 kV
400/5
P54x
Digital communication link
V02620

Figure 49: Typical three-terminal plain feeder circuit

If there are no VT inputs connected, the setting Phase Is1 must be 2.5 times the steady state charging current
suitably corrected for the different CT ratios as presented below in the calculation of charging current
compensated values. For an overhead line circuit with a charging current of 0.58A/km the charging current will be:
Ich = 0.58 A ( 45 + 30 + 10 ) = 49.3 A
If VT inputs are connected, there is a facility to overcome the effect of charging current. To do this you need to
enter the positive sequence capacitive susceptance value.
Considering the charging current on the circuit shown in the figure above, the following calculation applies:
Ich = 0.58 A ( 45 + 30 + 10 ) = 49.3 A
w C = Ich/V
Susceptance =
B = 49.3 A/( 275/
B = 0.31 x 10-3 S primary.
The CT ratios on the three ends are different, so it is necessary to apply a correction factor to ensure secondary
currents balance for all conditions:
To calculate the correction factor (CF), the same primary current must be used even if this current is not the
expected load transfer for every branch. This will ensure secondary current balance for all conditions.
CT ratio correction input data is shown in the table below:
Setting
Phase CT Primary
Ph CT Corr'tion
We recommend the following settings:
Phase Is1 = 0.2 p.u. = 0.2*1200A = 240A
Phase Is2 = 2 p.u. = 2*1200A = 2400A
130
45 km
Ch 1 Digital communication link
Ch 2
1200/5
End C
275 kV
P54x
Ch 1
Steady state charging current = 0.58 A/km - overhead line
Ö 3) kV primary
4000A
4000/1200 = 3.33
Protected line
10 km
Ch 2
End A
4000A
4000/1200 = 3.33
400/5
30 km
Ch 2
P54x
Ch 1
Digital communication link
End B
1200A
1 (reference)
P543i/P545i
End B
275 kV
End C
P54x1i-TM-EN-1